1. Field of the Invention
The present invention relates to a structure of an electron gun for a color cathode ray tube. More particularly, the invention relates to a structure of an electron gun for a color cathode ray tube capable of reducing a drive voltage by creating an optimum relation among a cathode, a first electrode, and a second electrode mounted in the electron gun, and preventing degradation in responsiveness to input signals on a high-resolution screen and a focus characteristic.
2. Background of the Related Art
FIG. 1 is an explanatory diagram of a structure of an electron gun for a cathode ray tube in a related art, and FIG. 2 is an explanatory diagram of an outward appearance of the electron gun in FIG. 1.
To give more details on the structure and functions of the electron gun for the cathode ray tube with reference to FIGS. 1 and 2, the electron gun includes three mutually independent cathodes 62, a first electrode (G1) 64 positioned spaced apart for a predetermined distance from the cathode 62, a second electrode (G2) 65, a third electrode (G3) 66, a fourth electrode (G4) 67, a fifth electrode (G5) 68, and a sixth electrode (G6) 69, the second through sixth electrodes being arranged at regular intervals from the first electrode 64 in a tube axis (or in-line) direction wherein a shield cup 70 with a bulb space contact (BSC) 71 adhered thereto is disposed on an upper portion of the last electrode, namely the sixth electrode 69 for electrically connecting the electron gun with a funnel 2 of the cathode ray tube as well as fixing the electron gun to a neck portion 2a of the funnel 2.
Also, a deflection yoke 4 for deflecting an electron beam 5 onto the entire screen is installed outside of the neck portion 2a of the funnel 2 where the electron gun is mounted on.
Based on the above construction, the electron gun emits electrons when a heater 63 built in the cathode 62 is heated up using the power supplied by a stem pin 61. The electrons existing as the beam shape (i.e. electron beam) are preliminarily converged by a pre-focus lens formed between the first electrode 64 and the second electrode 65, converged later by a pre-main lens formed by a potential difference among the third electrode 66, the fourth electrode 67 and the fifth electrode 68, and finally converged and accelerated when passing a main lens formed by a potential difference between the fifth electrode 68 and the sixth electrode 69.
Basically, an image is formed on the screen when the electron beams 5 are deflected onto the entire screen by the deflection yoke 4 and passes a shadow mask 3 at a predetermined distance from the panel 1 and strikes a fluorescent screen 1a formed on an inner surface of the panel 1.
FIG. 3 diagrammatically explains the structure of a part of the electron gun where the electron beam is formed.
As represented in FIGS. 1 through 3, the electron beam 5 is formed on the cathode 62, the first electrode 64 at a predetermined distance from the cathode 62, and the second through fourth electrodes 65, 66, and 67. Normally, the intensity of the electron beam 5 modulates in accordance with image signals applied from an external drive circuit 30, that is, red (Sr), green (Sg), and blue (Sb) colors.
As aforementioned, the first electrode 64 is disposed at a predetermined distance from the cathode 62. And, an electron beam passing hole (or through-hole) with a diameter D is formed on the first electrode 64.
In addition, a thing point 28 is formed between the first electrode 64 and the second electrode 65 where the two electrodes are spaced out by a constant distance B.
Upon application of a constant potential ranging from 400V to 1000V to the second electrode 65, the heater 63 heats the cathode 62, consequently emitting electrons therefrom. The emitted electrons are accelerated toward the first electrode 64 in which they form three electron beams 5, and these three electron beams 5 pass an electron beam passing hole 64A of the first electrode 64 and further an electron beam passing hole 65A of the second electrode 65. Later, these electron beams 5 are preliminarily converged by the pre-focus lens 40 formed between the second electrode and the third electrode 66 to which a 5 to 10 kV high voltage is applied.
The pre-focus lens 40 or the diameter of the pre-focus lens is controlled by the size of the electron beam passing hole 6A of the first electrode 64, the size of the electron beam passing hole 65A of the second electrode 65, a thickness T of the first electrode 64, and the gap B between the first and second electrodes 64 and 65.
Also, a pre-main lens 41 is formed between the third electrode 66 and the fourth electrode 67.
FIG. 4 illustrates another related art cathode ray tube particularly comprising a coining part in the second electrode for reinforcing the pre-focus lens effect.
For instance, a Japanese Patent Publication No. 1999-288664 discloses a cathode ray tube comprising a coining part 65C for adjusting the gap between the second electrode 65 and the third electrode 66, ther by reinforcing the pre-focus lens effect and preventing difficulty of assembly or deterioration in an assembly precision within a limited design system based on an automatic process not necessarily using additional parts.
More specifically, the second electrode 65 is provided with the coining part 65C with a diameter 2R and a thickness t in the vicinity of the electron beam passing hole 65A, and a slot part 65B with a predetermined thickness t1xe2x88x92t2 for improving a focus characteristic of the electron beam.
On the other hand, a drive voltage and a cutoff voltage are different as follows. Usually, the drive voltage from the external drive circuit 30 is applied to respective cathodes corresponding to three-color fluorescent substances through the stem pin 61. When the drive voltage varies, the variation synchronizes with deflection and resultantly the amount of the electron beam 5 emitted from each cathode 62 is controlled thereby. At this time, the voltage right before the electron beam 5 is emitted from the cathode 62 is called a cutoff voltage. Normally, the cutoff voltage is obtained when the brightness of the screen is at zero level (dark point).
To be short, the cutoff voltage clan be expressed by the following equation:
Cutoff=Kxc3x97S3/Cxc3x97Txc3x97B)xc3x97Vg2xe2x80x83xe2x80x83Equation (1)
In the equation, K is a proportional constant; S is an area of the electron beam passing hole 64A of the first electrode 64; C is a gap between the cathode 62 and the first electrode 64; T is a thickness of the electron beam passing hole 64A of the first electrode; B is a gap between the first electrode 64 and the second electrode 65; and Vg2 is an applied voltage to the second electrode 65.
Given the applied voltage to the second electrode 65 is 260V, the cutoff voltage for a color monitor cathode ray tube is approximately 55V.
According to Japanese Patent Publication No. 53-18866, a color cathode ray tube for a color television typically has a 0.6 mm diameter first electrode for the electron gun, and the drive voltage for a cathode ray tube particularly in a data processing monitor, e.g. a computer, is approximately 50V, and a current capacity a cathode emits is about 0.3 mA.
This corresponds when the screen of the cathode ray tube is at its recommended brightness level, namely 100 cd/m2.
When brightness, resolution and contrast values are substantially high, it is more likely to get a desirable display area for the color cathode ray tube.
Accordingly, as for the cathode ray tube for a monitor which requires all the above characteristics, one needs to reduce a beam spot size at a high brightness and increase the number of pixels, conforming to the increase in the resolution of a dot pitch of each color for composing the fluorescent screen and the elongation of a display screen.
To reduce the diameter of a beam spot more effectively, one makes the first electrode 64 or an electron beam passing hole of a neighboring electrode smaller and spaces electrodes at more optimal intervals, whereby the diameter of a projected thing point 28 is reduced and the current density of the cathode 62 is increased.
However, when more heat or thermal energy (joule) is applied, it increases the current density of the cathode 62, and this consequently causes electron-emitting substances like barium in a corresponding cathode 62 to be evaporated. In short, if the cathode capacity is deteriorated, the lifespan of the cathode ray tube is shortened as well.
In addition, high resolution of the fluorescent dot pitch and increased number of display screens (or frames) responsive to the screen elongation only deteriorate beam transmittance of a shadow mask. Although one tried to maintain desirable screen brightness by having the cathode emit more current, it only shortened the life of the cathode ray tube much faster.
In the meantime, in order to increase the display frames, frequency of the drive voltage for amplifying image signals applied to the cathode is often increased. In doing so, however, the drive voltage modulates the amplitude of image signals.
For instance, suppose that 2M pixel number corresponding to 1600 dots*1200 lines needs to be displayed out of 1.3M pixel corresponding to 1280 dots*1024 lines. Then, one needs to set a clock frequency of a video bandwidth to be in the range of 150-200 MHz.
However, there is a limitation in a circuit frequency characteristic to amplify the amplitude of an image signal to a designated drive voltage.
FIG. 5 illustrates a drive voltage for obtaining a desirable brightness for the screen. As shown in the drawing, the maximum amplitude of the drive voltage for obtaining the preferable screen brightness is approximately 50V given that the clock frequency of the video bandwidth falls in the range of 150-200 MHz.
Generally, a drive voltage can be expressed by the following equation:
Drive voltage=Cutoff voltagexe2x88x92cathode voltagexe2x80x83xe2x80x83Equation (2)
FIG. 6 graphically represents a relation between the current capacity and the drive/cutoff voltages. As shown in the drawing, given the same drive voltage, the current capacity is inversely proportional to the cutoff voltage. (i.e. The lower the cutoff voltage is, the more the current is emitted.)
That is, given the same drive voltage, more current is emitted when the cutoff voltage is 30V rather than 50V.
Applying the above to a more practical sense, one can easily think of a computer monitor whose brightness is relatively less brightness than that of a television in general. This problem has been remained unsolved regardless of the fact that the Internet and image media system have been rapidly developed within few decades, and more people are now watching animation through computer monitors rather than televisions.
To be more specific, the brightness of the cathode ray tube is normally dependent on the current capacity of the electron gun. As discussed before, when 260V is applied to the second electrode of the cathode ray tube for a monitor, the cutoff voltage typically ranges from 50V to 55V, which is obviously higher than 30V. Therefore, as demonstrated in FIG. 6, the current capacity emitted when the cutoff voltage is 50-55V is just a half of the current capacity emitted when the cutoff voltage is 30V. In consequence, the brightness was lowered and viewers had to stand watching relatively darker images.
Of course, some monitor manufacturers tried to increase the current capacity by applying an even higher drive voltage to the cathode. Unfortunately though, they had to pay more to increase the drive voltage at a high frequency of a color monitor.
FIGS. 7 and 8 are explanatory diagrams of delay in a reply signal to an input signal. For instance, FIG. 7 demonstrates that when the clock frequency is 150 MHz of a video bandwidth, a time delay occurs as the reply signal ascends or descends.
Similarly, FIG. 8 demonstrates that when the clock frequency is 200 MHz of a video bandwidth, a time delay occurs as the reply signal ascends or descends. One thing different from FIG. 7 is that the time de lay gets worse, compared to that of 150 MHz of the clock frequency, and at the same time some of the amplitude is lost, degrading the input signal.
In consequence, a more accurate input signal cannot be transferred to the cathode, and the advantages of the smaller beam spot are not necessarily reflected on the resolution.
In other words, it gets more difficult to display vertical lines that are directly influenced of a relatively high frequency, i.e. horizontal deflection frequency. As a result thereof, the brightness of the vertical lines gets worse and bright lines flow towards a scanning direction.
Meanwhile, a sufficient drive voltage is obtained for horizontal lines that are directly influenced of a relatively low frequency, i.e. vertical deflection frequency.
Nevertheless, this increases a brightness difference between vertical lines and horizontal lines and resultantly creates unnatural images.
Generally speaking on a drive characteristic of the color cathode ray tube, a cathode voltage is set low at the point of emitting electrons so as to reduce the amplitude of the drive voltage.
In such case, however, the current density at the cathode is also reduced. As such, the beam spot diameter on the screen gets longer and the resolution is degraded.
As an attempt to solve the above problems, Korean Patent No. 308366 disclosed an equation, D3xe2x89xa6(1.54 B+0.17)xc3x97T (See FIG. 3), wherein D is an average diameter of the electron beam passing hole 64A in the vertical and horizontal directions of the first electrode 64; B is a gap between the electron beam passing hole 64A of the first electrode 64 and the electron beam passing hole 65A of the second electrode 65; and T is a thickness of an electrode plate of the electron beam passing hole 64A of the first electrode 64.
Still a problem arose especially when the gap between the first electrode 64 and the second electrode 65 was increased to satisfy the above equation. That is, an emission angle at the thing point 28 was increased in such case, and this consequently made the size of the beam spot bigger.
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
Accordingly, one object of the present invention is to solve the foregoing problems by providing a structure of an electron gun for a color cathode ray tube capable of implementing a low cutoff voltage without incurring additional costs, thereby increasing a current capacity at a given drive voltage and obtaining a desired focus characteristic, namely satisfying a high resolution both in a high and low current areas.
Another object of the present invention to provide a structure of an electron gun for a color cathode ray tube capable of reducing a drive voltage by creating an optimum relation among a cathode, a first electrode, and a second electrode mounted in the electron gun, and preventing degradation in responsiveness to input signals on a high-resolution screen and a focus characteristic.
The foregoing and other objects and advantages are realized by providing the structure of an electron gun for a color cathode ray tube comprising a fluorescent screen, a shadow mask and the electron gun, in which the fluorescent screen includes a fluorescent film with three-color pixels arranged thereon; the shadow mask is a color selecting electrode positioned adjacent to the fluorescent screen; and the electron gun includes a cathode for emitting three electron beam, a first electrode and a second electrode; and the means for forming the main lens includes a plurality of electrodes for focusing the three electron beams onto the fluorescent screen, wherein an electron beam passing hole (or through-hole) of the first electrode ranges from 0.06 mm2 to 0.12 mm2, and a gap between the first electrode and the second electrode ranges from 0.12 mm to 0.3 mm.
Additional advantages, objects and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.